Candidate target mechanisms of the growth inhibitor cyromazine: Studies of phenylalanine hydroxylase, puparial amino acids, and dihydrofolate reductase in dipteran insects

Abstract

Cyromazine, an insect growth regulator, affects larval and pupal cuticles in dipterans and some other insects. The mode of action of this aminotriazine is not known yet, though it has been shown not to inhibit the synthesis of chitin and cuticular proteins. Cyromazine may, however, act on some step(s) of sclerotization of the cuticle. In the present study, we have analyzed the key enzyme for the production of sclerotization agents, phenylalanine hydroxylase (PAH), using the enzyme from Drosophila, a cyromazine-sensitive insect. PAH was studied in vitro with cyromazine and three biologically less active derivatives at concentrations ranging from 1 microM to 1 mM. None of the compounds did significantly affect PAH activity. Nor did cyromazine, fed to last instar larvae of Musca domestica, change the relative content of phenylalanine and tyrosine, or the total amount and profile of amino acids of puparial cuticles, which showed a larviform shape typical for cyromazine intoxication. Taken together, this study does not support the hypothesis that phenylalanine hydroxylase represents a target site of cyromazine. In additional studies, the conflicting results, as reported by others, on in vitro inhibition of dihydrofolate reductase (DHFR) by cyromazine were re-examined using the enzymes from larvae of the blowfly, Protophormia terraenovae, and from hen liver. There was no significant inhibition of either DHFR at 100 microM by cyromazine as well as by dicylanil, a pyrimidine analog that is biologically more active than cyromazine. In conclusion, the mode of action of cyromazine remains completely open.

The insect growth regulator insecticide cyromazine causes earlier emergence in Drosophila melanogaster

“…It has been suggested that cyromazine may interfere with nucleotide synthesis (El-Oshar et al, 1985;Binnington et al, 1987;Binnington and Retnakaran, 1991;Bel et al, 2000;Chen et al, 2006). One biological outcome following the peak in 20E The data presented in this study suggest a connection between a cyromazine mode of action and the hormonal control of larval development.…”

“…Cyromazine inhibits growth and development of larval life stages of Diptera. Despite numerous biochemical and some genetic studies, the precise mode of cyromazine action remains undetermined (Miller et al, 1981;Turnbull and Howells, 1982;Bel et al, 2000, Daborn et al, 2000, Chen et al, 2006.…”

“…Typically administered through feeding, the physiological effects of cyromazine poisoning observed in larvae include irregular sclerotization and melanization of the cuticle, necrotic lesions in the cuticle, rupture of the body wall, and ultimately lead to death (Binnington, 1985;Friedel et al, 1988;Vinuela and Budia, 1994). As the physiological effects of cyromazine are manifest in the cuticle, it has been hypothesized that cyromazine may target the cuticle (Miller et al, 1981, Binnington, 1985Friedel et al, 1988, Bel et al, 2000.…”